Alkaline cell with performance enhancing additives

ABSTRACT

Alkaline electrochemical cells having extended service life and acceptable gassing and corrosion properties are disclosed. An amphoteric surfactant can be incorporated into the gelled anode mixture of an alkaline electrochemical cell, optionally with an organic phosphate ester surfactant or a sulfonic acid type organic surfactant or both. Zinc particles having a defined distribution of particle sizes can also be incorporated into a zinc anode. The electrolyte included, in the anode mixture can have a reduced hydroxide concentration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. Ser. No. 11/739,507, filed Apr.24, 2007 entitled “Alkaline Cell with Performance Enhancing Additives”,which is a continuation of U.S. Ser. No. 10/648,134, filed Aug. 26, 2003entitled “Alkaline Cell with Performance Enhancing Additives”, now U.S.Pat. No. 7,226,696, which is a continuation-in-part of U.S. Ser. No.10/375,381, filed Feb. 27, 2003, now U.S. Pat. No. 7,169,504, which is acontinuation-in-part of U.S. Ser. No. 10/090,137, filed Feb. 27, 2002,now U.S. Pat. No. 6,872,489, both of which are entitled “Alkaline Cellwith Gassing Inhibitors”, the disclosures of which are herebyincorporated by reference as if set forth in their entirety herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not yet determined.

BACKGROUND OF THE INVENTION

Zinc anode gels of alkaline electrochemical cells are prone toelectrochemical corrosion reactions when stored at or above roomtemperature. The alkaline electrolyte in the anode gel corrodes the zincanode upon contact, forming oxidized zinc products that decrease theavailability of active zinc while simultaneously generating hydrogengas. The rate of corrosion tends to increase as the electrolyte is mademore dilute and as the storage temperature rises and can lead to asignificant decrease in cell capacity. Cell discharge performance, onthe other hand, can be improved making the electrolyte increasinglydiluted. It is thus desirable to suppress gas generation when usingdiluted alkaline electrolytes for increased performance. The additionalwater aids in the following cathodic reaction:2MnO₂+2H₂O+2e ⁻→2MnOOH+2OH⁻ (for MnO₂ cell)  (1)½O₂+H₂O+2e ⁻→2OH⁻ (for zinc-air cell)

However, lowering the hydroxide concentration in the electrolyte cancause the anode to become over-diluted and depleted in hydroxide ionswhich are needed to sustain the anodic cell reaction:Zn+4OH⁻→Zn(OH)₄ ²⁻+2e ⁻

The depletion of hydroxide ions can become prominent during medium andhigh continuous discharge rates and induce depressed cell performancedue to anode failure in these cases. Furthermore, when the electrolyteis saturated with zincate Zn(OH)₄ ⁻² produced in the above reaction, thezincate precipitates to form zinc oxide which, in turn, passivates thezinc anode, thereby lowering cell performance. Conventional zinc powderscontain particles having a wide distribution of particle sizes rangingfrom a few microns to about 1000 microns, with most of the particle sizedistribution ranging between 25 microns and 500 microns. To achieveproper discharge of such conventional zinc powders, a KOH concentrationabove 34% is conventionally used. At lower concentrations, insufficientKOH is available to the anode and can lead to passivation. Nevertheless,lower electrolyte concentrations are desired because of lower ionicresistance that brings about higher cell operating voltage.

Additionally, hydrogen gas generated during corrosion reactions canincrease the internal cell pressure, cause electrolyte leakage anddisrupt cell integrity. The rate at which the hydrogen gas is generatedat the anode zinc surface accelerates when the battery is partiallydischarged, thereby decreasing the battery's resistance to electrolyteleakage. The electrochemical corrosion reactions that lead to hydrogenevolution involve cathodic and anodic sites on the zinc anode surface.Such sites can include surface and bulk metal impurities, surfacelattice features, grain boundary features, lattice defects, pointdefects, and inclusions.

To minimize undesirable corrosion and gassing during storage, it istypical to employ corrosion-resistant zinc alloys and to reduce theextent of impurities in the anode. Additionally, organic surfactants andinorganic corrosion-inhibiting agents are commonly added to zinc anodes.Surfactants act at the anode-electrolyte interface by forming ahydrophobic film that protects the zinc anode surface during storage.The inhibitive efficiency of surfactants to increase the corrosionresistance of zinc depends on their chemical structure, concentration,and their stability in the electrolyte.

Among the surfactants known to be effective at controlling gassing areorganic phosphate esters such as the ethylene oxide-adduct typedisclosed by Rossler et al. in U.S. Pat. No. 4,195,120, incorporatedherein by reference. In U.S. Pat. No. 4,777,100, Chalilpoyil et al.disclosed an anode containing single crystal zinc particles with asurface-active heteropolar ethylene oxide additive including organicphosphate esters. In U.S. Pat. No. 5,378,559, Randell et al. disclose agalvanic cell that contains a phosphate ester compound to reduce gassingattributed to the brass anode current collector.

Despite their ability to control gel gassing and cell gassing, organicphosphate ester corrosion-inhibiting surfactants also typically decreasehigh rate discharge performance in electrochemical cells and canadversely affect intermittent cell discharge performance. It is believedthat discharge performance suffers as a result of anode failed caused bya combination of zinc passivation, hydroxide ion depletion, andreduction in hydroxide ion diffusion. Therefore, new approaches aresought for inhibiting corrosion and preventing leakage withoutsimultaneously reducing high rate cell discharge performance. At thesame time, it is also of interest to develop new classes ofcorrosion-inhibiting surfactants for use in gelled anodes of alkalineelectrochemical cells. Still further, extension of service life bychemical and physical modifications to the anode without sacrificingimprovements in corrosion resistance and electrochemical behavior arealso highly sought.

BRIEF SUMMARY OF THE INVENTION

The present invention is summarized in that acceptable gel gassinglevels are maintained when the gel of an alkaline electrochemical cellcontains a surfactant having the general formula Y SOx⁻ where preferablyx=3 or x=4 and Y preferably is selected from an alkyl group, an arylgroup, an alkylaryl group, and a carboxy acid group [(CH₂)_(n)—COO⁻], ora salt of any of the foregoing, where the number of carbon atoms, n, inan alkyl chain preferably ranges from 1 to about 26. To the knowledge ofthe inventors, sulfonated or sulfated acid type surfactants have notpreviously been employed in gelled anodes, but are shown herein toeffectively help inhibit discharge leakage and to maintain cellintegrity.

The invention is further summarized in that unexpected and advantageousbenefits are recognized when the aforementioned acid type surfactant orsalt thereof is used in combination with a phosphate ester surfactantthat can be added to the gelled anode in an amount ranging from 0.0003%to 0.02%. For example, cell reliability can be maintained and cellgassing can be suppressed.

The invention is further summarized in that unexpected and advantageousbenefits are recognized when the aforementioned acid type surfactant orsalt thereof is used in combination with a phosphate ester surfactantwhen zinc particles are added to the gelled anode. For example, cellperformance can be increased and cell gassing can be suppressed.

The invention is further summarized in that unexpected and advantageousbenefits are recognized when the aforementioned acid type surfactant orsalt thereof is used in combination with a phosphate ester surfactantwhen the gel contains an electrolyte having a hydroxide concentrationless than 40%. For example, cell performance can be increased and cellgassing can be suppressed.

The present invention is also summarized in that service life of a cellof the invention is improved by chemical modification to the anode.Accordingly, an anode for an alkaline electrochemical cell of theinvention can comprise at least one amphoteric fatty amine surfactant.An amphoteric surfactant has both positively and negatively chargedgroups and can function as an acid or as a base depending upon theenvironmental pH. An amphoteric surfactant suitable. for use in an anodeof the invention is compatible with the strong alkaline electrolyte usedin zinc manganese dioxide alkaline cells and can extend service life ofthe cells at both high and low rate discharge. A suitable amphotericfatty amine surfactant has the general structure of formula (I), shownin FIG. 1A, wherein R₁ can be an alkyl group having between 8 to 30unbranched carbon atoms; R₂ can be a short alkyl group having from 1 toabout 6 unbranched carbon atoms that can have one or more hydroxyl sidegroups; R₃ can be selected from a polyethylene oxide group havingbetween 3 to 40 ethylene oxide units and a polypropylene oxide grouphaving from 1 to 10 and, more preferably, from 2 to 5 propylene oxideunits; and X can be an anionic acid group, an anionic acid ester, or analkali metal salt of an anionic acid or acid ester, where the acid ispreferably sulfonic, carboxylic or phosphoric acid.

Preferred compounds having the general formula of Compound (I) includecomplex amine carboxylates, particularly sodium and potassium salts ofpolyethoxylated fatty amino carboxylates wherein R₁ contains between 18to 22 carbon atoms. A compound preferred by the inventors is a sterylamino polyethoxylated carboxylate commercially available from BASFCorporation in approximately 70 percent diluted form under the tradename MAFO® 13.

In another related embodiment, the invention is further summarized inthat an anode that comprises at least one amphoteric surfactant havingthe general formula of Compound (I) can optionally further comprise atleast one amphoteric surfactant having the general formula of compound(II), shown in FIG. 1B, wherein R₄ can be an unbranched alkyl grouphaving from 8 to 30 carbon atoms that forms an aliphatic fatty aminewhen bound to the nitrogen atom; R₅ can be selected from a polyethyleneoxide group having between 3 to 40 ethylene oxide units and apolypropylene oxide group having from 1 to 10 and, more preferably, from2 to 5 propylene oxide units; R₆, like R₅, can also be a polyethyleneoxide group or a polypropylene oxide group, but can also be a hydrogen.An additive having the structure of Compound II can be a tertiarypolyethoxylated fatty amine having two polyethylene oxide groupsattached to the amine nitrogen.

In a preferred aspect, the amphoteric surfactant in the anode comprisesa blend of one or more polyethoxylated fatty amino carboxylates and oneor more polyethoxylated fatty mines. An amphoteric surfactant mixturethat comprises compounds having the general formulas of Compounds (I)and (II) and which is suitable for use in an anode of the invention iscommercially available under the trade name MAFO® 13 MOD1 from BASFCorporation in approximately 90 percent diluted form. This commerciallyavailable amphoteric surfactant mixture is preferred by the inventorsfor use in anodes of the invention.

Whether a single amphoteric surfactant or a mixture of amphotericsurfactants is provided, a suitable amount of amphoteric surfactant inthe anode is between about 5 and about 1000 ppm relative to zinc weight.

In yet another aspect of the invention, an anode of the invention thatcontains an amphoteric surfactant (or an amphoteric surfactant mixture)as described above can optionally also contain either or both of theorganic acid type organic surfactant and the phosphate ester surfactantdescribed elsewhere herein, wherein the amphoteric surfactant inhibitscorrosion and extends service life of the cell, under both continuousand ANSI discharge conditions. Without intending to be limited to atheory of the invention, it is thought that the combination ofsurfactants increases availability of hydroxide ion reactants orinterferes with processes for forming a passivating film on the surfaceof the anodic zinc, thereby delaying zinc passivation and resultinganode failure.

In still another aspect of the invention, it is here disclosed thatphysical modifications to the anode can also improve cell service life,either alone or in combination with chemical modifications noted above.For example, one can efficiently discharge cells having anadvantageously lower concentration of hydroxide ions in the electrolytethan can be used in conventional cells by reducing diffusion resistancefor the hydroxide ions. This can be accomplished, for example, byadjusting the zinc particle size distribution to provide in the anode anarrow distribution of similar zinc particle sizes, thereby enhancingporosity (diffusion paths) for the hydroxide ions. In addition toimproving diffusion properties, the particle size distributions of thisinvention also provide the porosity sites for the precipitation of ZnO,thereby delaying anode passivation. This approach is effective for usein the anodes of both manganese dioxide and zinc-air alkaline cells andcan be used alone or in combination with other improvements disclosedherein.

In a related aspect, a suitable zinc particle size distribution is onein which at least 70% of the particles have a standard mesh-sievedparticle size within a 100 micron size range and in which the mode ofthe distribution is between about 100 and about 300 microns. Inparticular, particle size distributions meeting the above-noted testsand having a mode at 100 microns or at 150 microns or at 200 microns,each plus or minus about 10%, are advantageously used in the invention.It is preferred that 70% of the particles be distributed in a sizedistribution range even more narrow than 100 microns, for example 50microns or even 40 microns or less.

It is an object of the present invention to provide an alkalineelectrochemical cell in combination with either or both of zincparticles added to the anode, and an electrolyte having a hydroxideconcentration less than 40%.

It is a further object of the present invention to provide a class ofsurfactants for reducing gelled anode gassing in an alkalineelectrochemical cell in combination with either or both of zincparticles added to the anode, and an electrolyte having a hydroxideconcentration less than 40%.

It is an advantage of the present invention that, when zinc particlesare added to the gelled anode mix, gel gassing is controlled in analkaline electrochemical cell that includes a sulfonic acid typesurfactant in the gelled anode mix.

It is another advantage of the present invention that gel gassing iscontrolled and cell discharge performance is increased in an alkalineelectrochemical cell that includes both a sulfonated or sulfated organicacid type organic surfactant, an organic phosphate ester surfactant, andan electrolyte having a hydroxide concentration less than 40% in thegelled anode mix.

These and other aspects of the invention are not intended to define thescope of the invention for which purpose claims are provided. In thefollowing description, reference is made to the accompanying drawings,which form a part hereof, and in which there is shown by way ofillustration, and not limitation, a preferred embodiment of theinvention. Such embodiment does not define the scope of the inventionand reference must be made therefore to the claims for this purpose.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following figures are presented, in which:

FIG. 1A depicts a first suitable amphoteric surfactant for use in ananode of the invention.

FIG. 1B depicts an optional second suitable amphoteric surfactant foruse in an anode surfactant mixture of the invention.

FIG. 2A is a graph illustrating levels of gassing in an electrochemicalcell having varying amounts of Witconate and RM-510 added to its gelledanode mixture;

FIG. 2B is a graph illustrating the performance of an electrochemicalcell having varying amounts of Witconate and RM-510 added to its gelledanode mixture;

FIG. 2C is a graph illustrating the performance of an electrochemicalcell having various inhibitors added to its gelled anode mixture;

FIG. 3 is a 3-dimensional representation of cell longevity as a functionof KOH concentration in the electrolyte and the amount of inhibitoradded to the anode;

FIG. 4 is a graph illustrating the effect of the addition of RM-510,Witconate, and zinc fines to the anode of a cell on cell performance;

FIG. 5 is a graph illustrating the effect of the addition of RM-510 andWitconate to the anode of a cell on cell performance;

FIG. 6 is a graph illustrating cell performance as a function of theconcentration of zinc fines in the anode; and

FIG. 7 is a graph illustrating cell performance as a function of thehydroxide concentration of electrolyte used to pre-wet the separator;and

FIG. 8 is a graph illustrating cell performance of a function of a mixof inhibitors added to the anode.

FIG. 9 depicts performance of a cell having an anode that comprises anamphoteric surfactant mixture where the cell was discharged at 1 Amp(continuous).

FIG. 10 depicts performance of a cell having an anode that comprises anamphoteric surfactant mixture where the cell was discharged at 250 mAmp(continuous).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the use of a sulfonated or sulfatedorganic acid surfactant alone or in combination with an organicphosphate ester surfactant as well as with at least one amphotericsurfactant to extend the service life of an alkaline electrochemicalcell. Physical modification to the anode of an alkaline electrochemicalcell can provide still further service life enhancements. The chemicaland physical modifications can be provided alone or in combination toachieve a desired service life improvement without unacceptablesacrifice in gassing and discharge performance properties of the cell.In the present invention, the surfactant inhibitor(s) are added into thegelled anode of an alkaline electrochemical cell. The invention furthercontemplates that the amount of electrolyte present may be reduced, andzinc fine particles (also referred to herein as zinc fines) may be addedto the electrolyte, either alone or in combination with each other, andalone or in combination with the inhibitor(s) described herein (e.g.,phosphate surfactant and/or sulfonated organic surfactant) to produceimproved cell performance.

A gelled anode mixture, a gelled anode, and an electrochemical cellcontaining the gelled anode can have the structures of and can beprepared as described in U.S. Pat. No. 6,040,088, incorporated byreference herein as if set for in its entirety, except as detailedbelow.

A suitable gelled anode according to the invention comprises a metalalloy powder (preferably an alloyed zinc powder), a gelling agent and analkaline electrolyte. The skilled artisan can readily select a suitablezinc alloy powder and an alkaline electrolyte from those known to theart. It is noted that known gelling agents other than the sodiumpolyacrylate gelling agent described in the incorporated patent aresuitable for use in the present invention. Such gelling agents includecarboxymethyl cellulose, crosslinking-typed branched polyacrylate acid,natural gum, and the like.

A preferred sulfonated or sulfated organic acid type organic surfactanthas the general formula Y SOx⁻ where preferably x=3 or x=4 and Y ispreferably selected from an alkyl group, an aryl group, and alkylarylgroup, and a carboxy acid group [(CH₂)_(n)—COO⁻], or a salt of any ofthe foregoing, where the number of carbon atoms, n, in an alkyl chainranges from 1 to about 26. Among these surfactants, octadecanoic acids,such as oleic acid, and salts thereof are preferred. A particularlypreferred agent is a sodium salt of sulfated oleic acid commerciallyavailable under the trade name Witconate™ 1840X, Dyasulf 2031, Dymosol2031, Freedom SOA-70, and Freedom SOA-70WV. Other suitable sulfonatedaryl, alkyl aryl, or alkyl compounds include other Witconate surfactantssuch as Witconate 90FH, Witconate 93S, Witconate 605A, and Witconate1260.

The sulfated organic acid type surfactant is advantageously provided inthe gelled anode mix at a concentration ranging from between about0.001% to about 0.06% relative to the weight of the gelled anode (10 ppmand 600 ppm), and more preferably between about 0.002% and about 0.03%(20 ppm and 300 ppm). In the stated ranges, the sulfonated organicsurfactant, alone or in combination with an organic phosphate estersurfactant, reduced KOH concentrations and zinc fines added to theanode, provides desirable gel characteristics as described herein. Inthe stated ranges, and in the presence of a phosphate organic ester, thesulfonated organic surfactant tends to suppress overdischarge leakagewhile reducing gel gassing and improving on intermittent dischargeperformance. Overdischarge leakage is reduced by at least 20%, morepreferably by at least 50%, and most preferably to 0, relative to cellslacking these surfactants. When an organic phosphate ester surfactant isprovided in the gelled anode mix, it is present at a concentrationranging from between about 0.0003% and about 0.02% relative to theweight of the gelled anode (3 ppm and 200 ppm), and more preferablybetween about 0.0003% and about 0.005% (3 ppm and 50 ppm)

The invention further contemplates that the amount of inhibitor added tothe cell may be expressed as a function of the weight of metal (e.g.,zinc) in the anode. In particular, RM-510 is preferably added in therange of 0.0004% to 0.015% relative to the weight of metal in the gelledanode, and more preferably between 0.0004% and 0.0075%. Witconate ispreferably added in the range of 0.0015% and 0.09% relative to theweight of metal in the gelled anode, and more preferably within therange of 0.003% and 0.045%. Either or both inhibitor may be added to thegelled anode mix either alone or in combination in accordance with anyof the embodiments described in this disclosure, unless otherwisespecified.

EXAMPLES

Table 1 shows that the three-day gassing rate of gels containing thesulfonated surfactant and the phosphate ester in combination wasappreciably suppressed relative to that of surfactant-free gels or tothat of gels containing either surfactant alone. The gels of Table 1were supplemented as shown either with RM-510 to 0.0035%, with Witconate1840X to 0.0125%, or with both surfactants at the same concentrations,relative to the weight of zinc in the anode gel.

TABLE 1 Description of Three-day Gel Gassing Gel sample μl/g/day Noinhibitor 6.2 ± 1.0 RM-510 5.4 ± 0.9 Witconate 1840X 6.8 ± 0.6 Witconate1840X + RM-510 3.9 ± 0.5

Table 2 summarizes partial discharge cell gassing measured in LR03 cellsafter discharge at 7.5 ohm to a 1.0 volt end voltage and storage for twoweeks at 160° F. Overdischarge leakage was measured after continuousdischarge at 10 ohms for 48 hours followed by room temperature storagefor five weeks. Discharge performance of LR03 cells at 600 mA during 10seconds per minute for 1 hour a day, was improved over the dischargeperformance of cells containing the phosphate ester alone. The gelledanode of otherwise conventional alkaline LR03 cells contained RM-510 toabout 0.0035% either alone or in combination with Witconate 1840X toabout 0.0125%, the amount of both surfactants being measured relative tothe weight of zinc alloy in the anode.

TABLE 2 Partial 600 mA Pulse Overdischarge Discharge No-delay Leakage %Cell Gas Ml Cycles to 0.9 V Witconate + RM-510 20 0.36 ± 0.11 390.4 ±3.5 RM-510 60 0.50 ± 0.23 362.2 ± 13.8

Further, Table 3 shows that the discharge performance at 500 mA ofcontinuous discharge of the LR03 cells containing both surfactants inthe gelled anode was superior to that of the cells that contained onlythe phosphate ester additive. Table 3 also shows that the high-rateconstant current discharge performance after high temperature storagewas significantly better in the LR03 cells containing both surfactantsthan in cells containing the phosphate ester alone.

TABLE 3 500 mA 500 mA No-Delay 14 days at Minutes to 0.9 V Minutes to0.9 V Witconate + RM-510 52.6 ± 2.8 55.6 ± 1.7 RM-510 48.0 ± 0.9 53.0 ±1.7

Electrochemical cells described herein are generally of the type havinga positive current collector, and a cathode in contact with the positivecurrent collector. The cell includes a gelled anode including at least ametal alloy powder (which could be a zinc alloy powder), a gellingagent, and an alkaline electrolyte. A separator is disposed between thecathode and the anode, and a negative current collector is in electricalcontact with the anode.

FIG. 1A illustrates the gassing characteristics of a LR6 electrochemicalcell (size AA) having various amounts of Witconate in combination withRM-510. The results indicate that anode gel gassing is reduced asWitconate is increasingly added to the anode. FIG. 1B illustrates theperformance of the cell tested corresponding to data shown in FIG. 1A,and indicates that cell performance increases until reaching an optimalvalue before decreasing. Under the parameters of this test, a Witconateconcentration of 125 ppm relative to the gelled anode mixture achievedoptimal results, though any amount of Witconate present in the anodebetween 0 and 209 ppm increased cell performance compared to the controlcell that was inhibitor free. FIG. 1C illustrates that the addition ofWitconate and RM-510 in the anode of an electrochemical cell enablesincreased cell performance at 1 A of continuous discharge over a cellcontaining RM-510 alone, or a cell that is inhibitor free. Theperformance was measured using the amount of time needed for the cell tooutput 1 volt during discharge at 1 A.

It should be appreciated that electrochemical cells may be tested inaccordance with several methods under the American National StandardsInstitute (ANSI). These tests include determining cellperformance/longevity under situations of constant cell discharge, cellpulse discharge (i.e., repeated application of 1 A for a period of 10seconds carried out every minute over the period of an hour per day),and intermittent cell discharge (i.e., a continuous discharge forrepeated limited periods of time, for example one hour per day). Thepresent invention recognizes that the various embodiments of the presentinvention increase cell performance under one or more of these tests, aswill now be described.

For instance, FIG. 2 illustrates the ANSI results of a LR6 cellincluding zinc fines and RM-510 inhibitor that was tested at 3.9 Ohm 1Hr/day of discharge. In particular, the life of the cell is measuredunder various conditions of KOH concentration (measured as a percentageby weight of total electrolyte, i.e., the KOH concentration) and undervarious amounts of RM-510 added to the anode. It is observed that theservice life of the cell is increased as the concentration of KOH in theelectrolyte is decreased from 38% to 33%, which reduces anodepassivation. The present invention recognizes that advantages may beachieved in a cell whose electrolyte has a concentration of 30% to 40%.Furthermore, as is discussed in more detail below, cell performance maybe enhanced using an anode gel having a KOH concentration between 20%and 30%. It is further observed that, while the addition of RM-510 aloneis known to suppress gassing in the cell, overall cell performance isreduced as additional RM-510 is added. Nevertheless, the suppression ofgassing is desirable to reduce the risk of increased internal cellpressure, which could cause electrolyte leakage and disrupt cellintegrity.

Referring to FIG. 3 the performance of a LR6 cell having a KOHconcentration of 34% and RM-510 and Witconate added to the cell anodewas compared to a control cell having a KOH concentration of 34% andRM-510 added to the anode. The cells were discharged at several constantcurrent rates. The results indicate that reduction of KOH concentrationin the electrolyte, and the addition of RM-510 and Witconate to theanode increases cell performance compared to conventional cells.Performance was determined based on the amount of time needed for thecell to reach a discharge voltage of 0.8V at 2.2 and 3.9 ohm, and 0.9 Vat 250 mA and 500 mA.

The present invention recognizes that another factor that controls cellperformance relates to the surface area of the anode. Specifically,increasing the active anode electrode surface area provides sufficientactive reaction sites needed to keep up with the cathodic reaction athigh discharge rates. Accordingly, cells are provided having apredetermined amount of zinc particles (which may either be in the formof zinc or a zinc alloy) added to the anode gel. The present inventioncontemplates zinc particles less than about 75 microns (−200 mesh size),that is, particles that pass a 200 mesh screen size are present in theanode in an amount less than approximately 10%, by weight relative tothe total zinc in the anode (including coarse zinc particles), andpreferably within the range of 1% and 10%, and more preferably betweenthe range of 1% and 8%, and more preferably still within the range of 4%and 8%, it being appreciated that smaller particles further increase theeffective surface area of the anode. Mesh sizes are stated herein tospecify a range of particle sizes. For example, −200 mesh indicatesparticles smaller than 75 microns, while +200 mesh indicates particleslarger than 75 microns. Alternatively still, desirable results may beattained using an amount of zinc fines greater than 10%, while the zincparticles having a diameter between 75 and 105 microns (+75 and −140mesh size) may be present at anywhere between 1% and 50%, and morepreferably between 10% and 40%, by weight of total zinc present in theanode.

The present invention recognizes that multiple ranges of zinc particleshaving a diameter less than 105 microns (−140 mesh size) includingparticles between 75 and 105 microns (+200 and −140 mesh size) and zincfines less than 75 microns (−200 mesh size), may be used to increasecell performance. For instance, it has been discovered the anode mayinclude zinc particles between 75 and 105 micrometers, and that theadvantages in cell performance are enhanced when the anode gel has anelectrolyte (KOH) concentration less than 30%, and preferably between20% and 30%. When zinc fines have a size between the range of 20 and 75micrometers (+625 and −200 mesh size), and preferably between 38 and 75micrometers (+400 and −200 mesh size), cell performance is particularlyenhanced when the KOH concentration is between 30% and 40%, andpreferably between 33% and 38%. Yet another preferred range within therange of 20% and 40% KOH concentration is the range between 20% and 34%,and more preferably 25% and 33%, and more preferably still between 25%and 30%. A “low KOH concentration” as used in this disclosure refers toa KOH concentration within any of the above-stated ranges.

Even though improved cell performance has been correlated to thepreferred ranges of zinc fine sizes in combination with the low KOHconcentrations, one skilled in the art would also recognize the benefitsof the addition of zinc fines and reduction of KOH individually.Accordingly, the present invention includes within its scope theaddition of zinc fines to an anode gel having a size within any of theabove-stated ranges either individually or in combination with any ofthe above-stated KOH concentrations. Likewise, the present inventionincludes within its scope an anode gel having a KOH concentration withinany of the above-stated ranges either individually or in combinationwith zinc fines having a size within any of the above-stated ranges.Moreover, the above-stated zinc fines and/or KOH concentration may beimplemented in a cell either alone or in combination with the inhibitorsdescribed herein (e.g., a phosphate surfactant and/or sulfonatedsurfactant).

For instance, referring now to FIG. 4, the service life of LR03 cellswere tested and compared against a control cell having 4% fines added tothe anode without an inhibitor. The control cell also included aconcentration of 37% KOH. A first test cell included 4% zinc fines incombination with RM-510 inhibitor, while a second test cell included 8%zinc fines in combination with RM-510 inhibitor. A third test cellincluded the additives of the first test cell, but further included anoleic acid inhibitor. A fourth test cell included the additives of thesecond test cell, but further included an oleic inhibitor. All testcells included a concentration of 34% KOH. The cell performance wasmeasured as the amount of time that elapsed before a cell output voltageof 0.9V was measured.

The decreasing cell performance component associated with the additionof RM-510 overcame the increasing cell performance component associatedwith the decrease of KOH concentration, as the service life of the firsttest cell was slightly reduced compared to the control cell. However,the addition of 4% fines to the second test cell (for a total of 8%) incombination with a reduced KOH concentration overcame the disadvantagesof RM-510 and produced a cell having a greater service life than thecontrol cell. The third test cell, which was identical to the first testcell but for the addition of an oleic acid inhibitor, produced a servicelife greater than both the first and second test cells. The fourth testcell, which was identical to the second test cell but for the additionof an oleic acid inhibitor, produced a service life greater than allother test cells. Accordingly, a cell having added zinc fines mayachieve improved performance over a comparable cell without zinc fines,and the performance may be further enhanced when an oleic acid inhibitoris added to the anode.

FIG. 5 illustrates the results of various ANSI tests performed on acontrol cell and a test cell. The control cell had a KOH concentrationof 37% and less than 5% zinc fines commonly present in the anode. Thetest cell had a KOH concentration of 34% and 8% zinc fines added to theanode. Both cells included a phosphate surfactant to suppress gassing.The separator was pre-wetted with 37% KOH concentration electrolyte forboth cells. Both cells were tested at 1 A continuous, 1 A pulsed, 3.9ohms at 1 hour/day, 250 mA at 1 hour/day, 10 ohm at 1 hour/day, and 43ohm at 4 hours/day. In all cases, the test cell performed better thanthe control cell, thereby indicating that the reduction of KOHconcentration from 37% and the addition of zinc fines increased cellperformance. Cell performance was determined based on the amount of timethat had elapsed for the cell to reach specific discharge voltagesdepending on the test. For instance, 1V was used for 1 A continuousdischarge; 0.9V was used for 1 A pulse, 0.8V was used for 3.9 ohm 1hr/day, 0.9V was used for 10 ohm 1 hr/day, and 0.9V was used for 43 ohm4 hr/day.

FIG. 6 illustrates that cell performance may be increased further byreducing the KOH concentration in electrolyte that is used to pre-wetthe separator of the cell. In particular, a control cell and a test cellwere tested under the conditions described above with reference to FIG.5. The control cell and test cell both included 8% zinc fines, and 34%KOH concentration in the anode. The separator of the control cell waspre-wetted with electrolyte having a 37% KOH concentration, while theseparator of the test cell was pre-wetted with electrolyte having a 34%KOH concentration. The increased cell performance of the test cell maythus be attributed to the decreased KOH concentration used to pre-wetthe separator. The present invention contemplates that a KOHconcentration within the range of 20% and 40%, and more preferably30%-40%, and more preferably still 33% and 38% used to pre-wet theseparator will produce increased cell performance with decreasing KOHconcentrations. A KOH concentration between 20% and 30%, and morepreferably between 25% and 30% to pre-wet the separator may alsoincrease cell performance. Cell performance is particularly increasedwhen the cell is discharged intermittently or during a pulsed discharge,as excess water is available for the cathodic reaction, and hydroxideions are thus replenished during the rest period of the cell to fuel theanodic reaction. Cell performance in FIG. 6 was determined based on thecriteria discussed above with reference to FIG. 5.

While additional reaction sites at the anode tend to produce increasedgassing, the addition of sulfonated or sulfated organic acid sufactantalone or in combination with an organic phosphate ester surfactantreduces the gassing, thereby increasing cell performance. Nevertheless,the skilled artisan appreciates that the present invention may include,either alone or in combination, the KOH concentrations described herein,the addition of zinc particles as described herein, and the addition ofsulfonated or sulfated organic acid surfactant alone, or in combinationwith an organic phosphate ester surfactant, to the anode, to improvecell performance as described throughout this disclosure.

FIG. 7 illustrates the performance of an LR6 test cell having a KOHconcentration of 34% in the anode gel, a separator pre-wetted with 34%KOH, and a mixture of RM-510 (at 12 ppm relative to the anode gel) andWitconate (at 125 ppm relative to the anode gel) added to the anode gel.The test cell was discharged, and its performance compared to a controlcell having 34% KOH in the gel, and a separator pre-wetted with 34% KOH,but with no Witconate added to the gel. Both the test cell and thecontrol cell included an anode having 8% zinc fines. The cellperformance, as determined by the amount of time needed for the cell tooutput 0.9V during continuous discharge, was found to increase when themixture of inhibitors was used with a reduced KOH concentration. FIG. 7thus shows that the addition of sulfonated or sulfated organic acidsurfactant in combination with the use of an organic phosphate estersurfactant suppresses what was previously thought to be adverse effectsof a low KOH concentration on continuous discharge performance, withoutadversely affecting the intermittent and pulse discharge performancegains observed at low KOH concentrations, as shown in FIG. 6. The use ofrelatively low concentrations of KOH in the anode and pre-wet solutions(within any of the above-stated ranges) improves performance at high andlow intermittent discharge rates.

As shown in FIGS. 9 and 10, respectively, otherwise conventional LR6zinc manganese dioxide cells containing a mixture of amphotericsurfactants (MAFO 13 MOD1) at 65 ppm in the anode relative to zincweight were discharged at 1 Amp (continuous) or at 250 mAmp (continuous)and in both cases a service life enhancement of at least about 20% wasobserved relative to conventional LR6 cells without the amphotericsurfactants.

Performance at 3.9 Ohm (continuous discharge) of LR6 cells containingthe MAFO 13 MOD1 mixture of surfactants (30 ppm relative to gel weightincluding zinc at between 66 and 70%) was enhanced by between about 15and 25% when 60 ppm Witconate 1840X surfactant was also blended into theanode. Similarly, performance at 3.9 Ohm (continuous discharge) of LR6cells containing the MAFO 13 MOD1 mixture of surfactants (30 ppmrelative to gel weight) was enhanced by between about 5 and 20% when 24ppm RM-510 surfactant was also blended into the anode. Similar resultswere observed at 360 mAmp discharge (continuous). In both cases, theobserved improvements were dependent upon concentration of surfactants,such that discharge performance could be varied by adjusting theconcentration of the various surfactant components in the anode.

Zinc oxidation rate was also improved when either the amphotericsurfactant mixture alone, the mixture with Witconate 1840X, or Witconate1840X plus RM-510 were provided in the anode. Zinc oxidation rate,measured by anodic polarization of zinc metal in the indicatedenvironments, was depressed relative to environments lacking inhibitors.A depressed oxidation rate indicates a suppressed tendency to metaldissolution. Thus, corrosion and gassing are anticipated to be low,relative to inhibitor-free environments, in these environments. At bothhigh and low continuous discharge rates, the metal dissolution stageduring zinc oxidation also slow downs in the indicated environments,resulting in suppressed consumption of hydroxide ions to form solublezincate and a slow down in zincate saturation. Thus, anode failure dueto zincate saturation and oxide precipitation is delayed when a suitableinhibitor is provided to extend service life.

It was also observed that while average discharge performance at 3.9ohm, 2.2 ohm and 360 mA (continuous) steadily decreased with increasingKOH concentration (31%-34%) in the electrolyte in the presence of 30 ppmof MAFO 13 MOD1, inclusion of 100 ppm Witconate with the MAFO 13 MOD1 inthe anode yielded a 2% to 6% steady increase with increasing KOHconcentration over and above the average performance of the highestperforming cell containing the MAFO 13 MOD 1 surfactant alone (31% KOH).

It was further observed that the rate of zinc oxidation as determined byzinc polarization in 34% KOH is lower when the electrolyte contains theamphoteric surfactant mixture at 60 ppm, a combination of RM-510 andWitconate 1840X at 24 and 125 ppm, respectively, or a combination ofWitconate 1840X and the amphoteric surfactant mixture at 30 ppm and 60ppm respectively.

Anode passivation was compared at 1 Amp (continuous) discharge of zincmanganese dioxide alkaline cells with either an anode having a standardzinc particle size distribution at 66% zinc loading and either (1) 37%KOH electrolyte with 2% zinc oxide, (2) 34% KOH electrolyte with 2% zincoxide, or (3) 30% KOH electrolyte with 2% zinc oxide. The anodic zinctended to passivate quickly (40 minutes) at KOH concentrations below37%, whereas longer discharge to endpoint was observed with conventionalelectrolyte. On the other hand, when zinc particles having a particlesize distribution with a mode at 100 microns, was substituted for thestandard zinc, discharge to endpoint at the reduced KOH concentrationswas as good or better than the standard. The 100 micron zinc powdershowed significantly less tendency to passivate at low electrolyteconcentrations and anodes containing the 100 micron zinc powder showedsignificantly higher operating voltage and improved service life ascompared to regular anodes.

Table 4 compares the performance in 100, 250 and 1000 mA continuousdischarge tests of a conventional zinc anode with 37% KOH, 2% zinc oxideand an anode having the 100 micron zinc anode with 34% KOH, 2% zincoxide).

TABLE 4 Performance to 1.1 V Performance to 0.9 V 100 um Powder 100 umPowder (PSD1) (PSD1) Control % of Control % of Test Min Min Control MinMin Control 100 mA 1165.6 1178.3 101.1% 1410.3 1406.3 99.7% Continous250 mA 334.9 346.2 103.4% 467.3 458.3 98.4% Continuous 1000 mA 22.1 26.7120.6% 51.3 55.0 107.3% ContinuousThe inventive zinc particle size distribution was also tested in anodesof cylindrical zinc-air cells at about 66% to 72% zinc loading. Theelectrolyte concentration was varied from 28% to 37%.

When cylindrical zinc-air anodes having a zinc particle sizedistribution with a mode at about 100 microns (68% zinc loading, 28%KOH, 2% zinc oxide) were compared to anodes containing zinc particleshaving a conventional size distribution (70.5% zinc loading, 31% KOH, 2%zinc oxide), longer running times were observed for inventive anodes.Performance was 7% higher than control in the 0.95 watts continuous testand 15% higher in the 0.952 watts/1 min-0.286 watts/9 min pulse test.The operating voltage is also 30-50 mV higher than control. Superiorresults were also obtained when the inventive zinc anodes were testedagainst cylindrical zinc-air cells employing a conventional zincparticle distribution in the anode with 68% zinc loading and 31% or 28%KOH concentration.

The invention has been described in connection with what are presentlyconsidered to be the most practical and preferred embodiments. However,the present invention has been presented by way of illustration and isnot intended to be limited to the disclosed embodiments. Accordingly,those skilled in the art will realize that the invention is intended toencompass all modifications and alternative arrangements included withinthe spirit and scope of the invention, as set forth by the appendedclaims.

We claim:
 1. A gelled anode mixture comprising: a metal alloy powdercomprising zinc alloy particles, wherein between about 4 weight % andabout 8 weight % of the zinc alloy particles have a particle size ofless than about 75 micrometers, a gelling agent, an alkaline electrolytehaving a hydroxide concentration between about 25 weight % and about 33weight %, and a phosphate ester surfactant present at a concentrationranging substantially between about 0.0004 weight % and about 0.0075weight % relative to the gelled anode mixture.
 2. A gelled anode mixtureas claimed in claim 1, wherein about 8 weight % of the zinc alloyparticles have a particle size of less than about 75 micrometers.
 3. Agelled anode mixture as claimed in claim 1, wherein between about 4weight % and about 8 weight % of the zinc alloy particles have aparticle size between about 20 micrometers and about 75 micrometers. 4.A gelled anode mixture as claimed in claim 1, wherein the hydroxideconcentration is between about 25 weight % and about 30 weight %.
 5. Agelled anode mixture as claimed in claim 1, wherein the electrolytecomprises KOH.
 6. A gelled anode mixture as claimed in claim 1, whereinthe surfactant is present at a concentration of about 0.0024 weight %relative to the gelled anode mixture.
 7. A gelled anode mixture asclaimed in claim 1, wherein about 8 weight % of the zinc alloy particleshave a particle size of less than about 75 micrometers, wherein thehydroxide concentration is between about 25 weight % and about 30 weight%, wherein the electrolyte comprises KOH, and wherein the surfactant ispresent at a concentration of about 0.0024 weight % relative to thegelled anode mixture.
 8. An alkaline electrochemical cell comprising: apositive current collector; a cathode in contact with the positivecurrent collector; a gelled anode mixture comprising a metal alloypowder comprising zinc alloy particles, wherein between about 4 weight %and about 8 weight % of the zinc alloy particles have a particle size ofless than about 75 micrometers, a gelling agent, an alkaline electrolytehaving a hydroxide concentration between about 25 weight % and about 33weight %, and a phosphate ester surfactant present at a concentrationranging substantially between about 0.0004 weight % and about 0.0075weight % relative to the gelled anode mixture, a separator between thecathode and the anode; and, a negative current collector in electricalcontact with the anode.
 9. An alkaline electrochemical cell as claimedin claim 8, wherein about 8 weight % of the zinc alloy particles have aparticle size of less than about 75 micrometers.
 10. An alkalineelectrochemical cell as claimed in claim 8, wherein between about 4weight % and about 8 weight % of the zinc alloy particles have aparticle size between about 20 micrometers and about 75 micrometers. 11.An alkaline electrochemical cell as claimed in claim 8, wherein thehydroxide concentration is between about 25 weight % and about 30 weight%.
 12. An alkaline electrochemical cell as claimed in claim 8, whereinthe electrolyte comprises KOH.
 13. An alkaline electrochemical cell asclaimed in claim 8, wherein the surfactant is present at a concentrationof about 0.0024 weight % relative to the gelled anode mixture.
 14. Analkaline electrochemical cell as claimed in claim 8, wherein about 8weight % of the zinc alloy particles have a particle size of less thanabout 75 micrometers, wherein the hydroxide concentration is betweenabout 25 weight % and about 30 weight %, wherein the electrolytecomprises KOH, and wherein the surfactant is present at a concentrationof about 0.0024 weight % relative to the gelled anode mixture.